Amplitude compressing/expanding circuit

ABSTRACT

In an amplitude expanding/compressing circuit for expanding, compressing, or expanding and compressing an amplitude of an input signal, the following are provided: 
     a control signal generator which receives an input signal and generates a control signal from said input signal; 
     a signal multiplier for multiplying the control signal and the input signal to generate a multiplied output; 
     a first subtracter for subtracting the input signal from the multiplied output to generate a first subtracter output signal; 
     a signal processor which receives the first subtracter output signal and generates a processed output from said first subtracter output; and 
     a second subtracter for subtracting the processed output from the multiplied output to obtain an amplitude expanded, amplitude compressed, or amplitude expanded and compressed output signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an amplitude compressing/expandingcircuit which is suitable for use in signal processing circuits ofelectronic appliances such as a compact disc (CD) player or a taperecorder.

2. Prior Art

In digital audio signal reproducing apparatus such as CD players anddigital audio tape recorders (DAT), an audio signal is digitized andrecorded on a recording medium, then reproduced and demodulated into acorresponding analog signal, whereby, a high-quality audio signal withlittle noise over a wide dynamic range is obtained.

An audio signal obtained from such digital audio signal reproducingapparatus, however, has an undesirably wide dynamic range.

Because of the unnecessarily wide dynamic range, the signal waveform ofthis audio signal, when recorded by an analog tape recorder, isdistorted at its higher signal level portion or has a deteriorated SN(signal-to-noise) ratio at its lower signal level portion.

Further, when a CD player or the like plays back such a signal in anenvironment where there is background noise, such as within anautomobile, there arises another problem in which the signal portion atthe lower signal level is lost in the background noise and becomesinaudible, or, conversely, the signal portion at the higher signal levelproduces an excessively large volume of sound.

To solve such conventional problems, one of the better solutions is toreduce the dynamic range of the audio signal obtained from the digitalaudio signal reproducing apparatus.

As shown in FIG. 1 for an input signal derived from an audio signalconverted into an analog signal, the input signal at levels lower than-30 dB of the peak level is transformed to an output signal having asignal level which varies in proportion to the variation in the inputsignal level, the constant of proportion being 1.

For the range higher than -30 dB, the input signal also may betransformed to an output signal having a signal level which varies inproportion to the variation in the input signal level, with, however, aconstant of proportion, for example, of 1/2.

By so doing, an input-output characteristic having ratios of amplitudecompression (CR) of 1 at levels below the -30 db point and 2 at levelsabove the -30 db point is obtained (hereinafter such point, -30 db inthe example, will be referred to as a threshold point, or level). As aresult the dynamic range of the audio signal as a whole can be madenarrower.

A prior art method using an amplitude compressing/expanding circuit 1 ofthe structure shown in FIG. 2 achieves such an amplitude compressing andexpanding effect.

An input signal S_(I) is input to an amplitude modulation circuit 3through a delay circuit 2 and to a control signal generator circuit 4.

The control signal generator circuit 4 detects the signal level of theinput signal and, based upon the result of the detection, outputs to theamplitude modulation circuit 3 a control signal S_(G) having a signallevel which changes in accordance with the signal level of the inputsignal S_(I).

The amplitude modulation circuit 3 is constructed of a multipliercircuit, such as a VCA (voltage controlled amplifier) or the like. Byhaving the input signal S_(I) amplitude-modulated by the control signalS_(G), an output S_(O) is obtained having a signal level correspondingto the input signal S_(I), but changed according to the signal level ofthe control signal S_(G).

The delay circuit 2 is provided so that no overshoot may occur in theoutput signal S_(O).

Representing the amplitude of the input signal S_(I) by "x" and theamplitude of the output signal S_(O) by "y", y can be expressed as afunction of x and the amplitude compression ratio CR:

    y=x.sup.1/CR                                               (1)

For an amplitude compression ratio CR of 1 as when below the thresholdpoint, the amplitude y is expressed as: ##EQU1##

The amplitude compression ratio CR of value 1 in the region below thethreshold point can be obtained by generating control signal S_(G) sothat the gain "g" in the amplitude modulation circuit 3 equals 1, asexpressed by

    G=1.                                                       (3)

In contrast thereto, to obtain an amplitude compression ratio CR ofvalue 2 as desired for when applied to the amplitude x of the inputsignal S_(I) in the range above the threshold point, the amplitude "y"of the output signal S_(O) and the gain "g" in the amplitude modulationcircuit 3 are given by ##EQU2##

Therefore, an input/output characteristic providing the amplitudecompression ratio CR having value of 2 for the range above the thresholdpoint can be obtained by outputting the control signal S_(G) so that thegain g in the amplitude modulation circuit 3 may become x^(-1/2).

To provide such a characteristic, the control signal generator circuit 4may be structured, for example, as shown in FIG. 3. As illustrated theinput signal S_(I) is supplied through an absolute value circuit 5 to anenvelope detector circuit 6 to produce a detection signal S_(L)proportional to the signal level of the input signal S_(I). This signalis then subjected to logarithmic conversion in an logarithmic convertercircuit 7 and output to an adder circuit 9.

The adder circuit 9 sums the logarithmically-converted detection signalS_(L) and a threshold point signal S_(H). A clipping circuit 8 receivesthe output of the adder circuit 9 and clips the summation signal below a0 value and outputs the clipped signal Sc_(CL) to a multiplier circuit10.

Thus, because the input signal S_(I) is determined by the thresholdpoint signal S_(H), a clipped signal S_(CL) is obtained which varieswith respect to the input signal S_(I) by setting the threshold pointsignal S_(H) to a predetermined value.

The multiplier circuit 10 receives both the clipped signal S_(CL) and acompression ratio controlling signal Sp and outputs a product signalthereof to the amplitude modulation circuit 3 through an exponentialconverter circuit 11.

By expressing the amplitude of the input signal S_(I) at the thresholdpoint (-30 dB in the present case) as "Y" and the amplitude of theoutput signal from the logarithmic converter circuit 7 as H when theinput signal S_(I) is at its peak level, the threshold point signalS_(H) is expressed as

    S.sub.H =H-Y/20 log.sub.e 10                               (6)

Accordingly, a clipped signal S_(CL) is obtained having a signal levelwhich varies with the signal level of the input signal S_(I) only whenthe input signal S.sub. is larger than the signal level at the thresholdpoint.

As a result, in the range below the threshold point, the clipped signalS_(CL) clipped at the 0 value through the clipper circuit 8 is obtained,and a control signal S_(G) is output through the exponential convertercircuit 11 having a value of 1 corresponding to the 0 value.

Consequently, the gain of the multiplier circuit 3 is limited to 1 andan output signal S_(O) is obtained having an amplitude compression ratioCR equal to 1 as shown in equation (2). Thus, the input/outputcharacteristic for producing an amplitude compression ratio CR of 1 isobtained.

For the range above the threshold point, the input/output characteristicfor producing an amplitude compression ratio of 2 will be satisfied ifthe gain of the multiplier circuit 3 is arranged to become x^(-1/2), asindicated in equation (5).

To do so a control signal S_(G), expressed as

    S.sub.G =X.sup.(1-CR)/CR                                   ( 7)

may be output with respect to the input signal S_(I).

More particularly, if the compression ratio controlling signal Sp issupplied to the multiplier circuit 10 so that the gain therein isexpressed as

    Ap=-(CR-1)/CR=-1/2                                         (8)

an output signal S_(O) can be obtained whose amplitude is compressed bythe compression ratio CR (CR=2 in the present case) in the range abovethe threshold point.

Thus, an amplitude compressing/expanding circuit 1 can generate aninput/output characteristic which changes the amplitude compressionratio CR from 1 to 2 when the signal level of the input signal S_(I) isincreased across the signal level at -30 db.

However, in the above-described conventional circuit arrangement, theamplitude compression ratio CR abruptly changes between the front rangeand the rear range of the threshold point, and therefore, when the audiosignal is reproduced in the conventional amplitude compressing/expandingcircuit 1, the reproduced sound is extremely unnatural to listeners.

To solve such problem, one conventional method has been proposed inwhich the amplitude compression ratio CR is gently changed by graduallychanging the signal levels of the threshold point signal S.sub. and thecompression ratio controlling signal Sp in accordance with a change inthe signal level of the input signal by using, for example, a ROM (readonly memory) table.

Another conventional method has been proposed in which the thresholdpoint signal S_(H) and the compression ratio controlling signal Sp arecontrolled by a control circuit having a processing circuit arrangementinstead of ROM table.

However, if such conventional methods are used, the construction of theamplitude compressing/expanding circuit as a whole becomes complex.Therefore, such methods are not yet satisfactorily practicable in therecent technology.

Further, because the output signal S_(O) of the above-describedamplitude compressing/expanding circuit is only obtained as a multipliedoutput signal S_(O) depending upon the control signal S_(G) providedfrom the control signal generator circuit 4, the operatingcharacteristic cannot be variable in accordance with the frequencies ofthe input signal.

The capability of making the operating characteristic of an amplitudecompressing/expanding function variable with the frequencies of theinput signal is widely applicable to the arts for handling audiosignals. For example, the input signal is applied to the compressedamplitude audio signal to compensate for the lack of the dynamic rangethat can be heard.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide animproved amplitude expander and/or compressor circuit which is free fromthe above-mentioned drawbacks encountered in the prior art circuit.

It is another object of this invention to provide an improved amplitudeexpander and/or compressor circuit in which a desired input/outputcharacteristic can be obtained with a relatively simple circuitconstruction.

It is another object of the present invention to provide an improvedamplitude expander and/or compressor circuit having an input/outputcharacteristic which can be controlled in accordance with thefrequencies of an input signal.

It is another object of this invention to provide an improved amplitudeexpander and/or compressor circuit in which an input signal can benormalized at high speed with a simple circuit construction.

It is another object of this invention to provide an improved amplitudeexpander and/or compressor circuit in which a smooth change in theamplitude compression ratio near the threshold level of the input signalcan be obtained with a simple circuit construction.

It is another object of this invention to provide an improved amplitudeexpander and/or compressor circuit in which the amplitude compressionratio is changed smoothly even with a plurality of amplitude compressionchanging points.

These and other objects of the invention are accomplished by anamplitude compressing/expanding circuit in which the amplitudecompression ratio for the output signal is switchable by controlling thechange in the gain of an amplitude modulating circuit based upon thesignal level of the input signal or the signal level of an output signalfrom the amplitude modulating circuit. The amplitudecompressing/expanding circuit includes an adder circuit for outputting adifference signal between the input signal and output signal and asignal processing circuit for generating an output signal whose signallevel varies with the difference signal. The amplitudecompressing/expanding circuit is adapted to output a summation signal ofthe output signals from the amplitude modulating circuit and the signalprocessing circuit.

In accordance with the amplitude compressing/expanding circuit thedifference signal between the input signal and the output signal of theamplitude modulating circuit is input to processing circuit andprocessed to generate a signal to be added to the output to generate asummation signal. By selecting a suitable input/output characteristic ofthe processing circuit, the summation signal may achieve a morepreferable input/output characteristic than that of the output signal.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of these and other objects of the presentinvention, reference is made to the following detailed description ofthe invention read in conjunction with the following drawings, in which:

FIG. 1 is a characteristic curve diagram used for explaining problems inthe prior art amplitude compressing/expanding circuit;

FIGS. 2 and 3 are schematic block diagrams of a conventional amplitudecompressing/expanding circuit;

FIG. 4 is a schematic block diagram of an amplitudecompressing/expanding circuit according to the present invention;

FIGS. 5 through 12 are characteristic curve diagrams used for explainingvarious operations of the amplitude compressing/expanding circuit shownin FIG. 4;

FIG. 13 is a characteristic curve diagram showing a loudness curve;

FIGS. 14 and 15 are characteristic curve diagrams used for explainingoperations of amplitude compressing/expanding circuits according toother preferred embodiments;

FIG. 16 is a schematic block diagram of one example of the controlsignal generator shown in FIG. 4;

FIG. 17 is a characteristic curve diagram used for explaining operationsof the amplitude compressing/expanding circuit shown in FIG. 16;

FIGS. 18 and 20 are schematic block diagrams for showing a portion ofthe circuit shown in FIG. 16;

FIGS. 19 and 21 are characteristic diagrams used for describing theoperation of the control signal generator illustrated in FIG. 16;

FIGS. 22 and 23 show data examples used for explaining the operation ofthe control signal generator shown in FIG. 16;

FIG. 24 is a schematic block diagram of a control signal generatoraccording to another preferred embodiment;

FIG. 25 is a circuit diagram of a portion of the amplitudecompressing/expanding circuit shown in FIG. 4;

FIG. 26 is a schematic block diagram for illustrating an equivalentcircuit for the circuit shown in FIG. 25;

FIG. 27 to 30 are characteristic curve diagrams used for explaining theoperations of the circuit shown in FIG. 25;

FIG. 31 is a schematic block diagram of an amplitudecompressing/expanding circuit according to another preferred embodimentof the invention;

FIG. 32 is a schematic block diagram of an equivalent circuit for thecircuit shown in FIG. 31;

FIG. 33 is a schematic block diagram of an amplitudecompressing/expanding circuit according to a modified preferredembodiment of the invention; and,

FIG. 34 is a characteristic curve diagram used for explaining operationsof the circuit shown in FIG. 33.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

One preferred embodiment according to the invention will now bedescribed with reference to the accompanying drawings.

It should be noted that the same reference numerals shown in FIG. 2 willbe employed as those for denoting the same or similar circuit elementsshown in FIG. 4.

FIG. 4 shows an amplitude compressing/expanding circuit 20 having aninput signal S and an amplitude compressed and/or expanded output signalS₀₁. The input signal S_(I) is input to a control signal generator 4, amultiplier circuit 3, and a subtracter circuit 21. The control signalgenerator 4 produces a control signal S_(G) as a function of the inputsignal S_(I), and outputs the control signal to the multiplier circuit 3and signal processing circuit 26. The input signal S_(I) and the controlsignal S_(G) are multiplied at the multiplier circuit 3 to generate amultiplied output signal S_(O). The input signal S_(I) is subtractedfrom the multiplied output signal S_(O) at subtractor circuit 21 togenerate a difference signal S_(R). The difference signal S_(R) is inputto a multiplier circuit 22 of the signal processing circuit 26.

The multiplier circuit 22 multiplies the control signal S_(G) and thedifference signal S_(R) to produce a product (multiplied) output signalwhich then is filtered by a filter circuit 23, and amplified by anamplifier circuit 24 and output from the signal processing circuit 26 asa processed signal.

When the multiplier circuit 22 is operated with its gain set below 1, soas to perform an amplitude compressing operation, the processed signalis an output signal S_(S) having a phase in inverse relationship withthe phase of the output signal S_(O). The output signals S_(O) and S_(S)are subtracted at an adder circuit 25 to generate a summation signalS_(O1).

The amplitude compressing/expanding circuit 20 according to onepreferred embodiment thus includes a multiplier circuit 3 for amplifyingthe input signal S_(I) by a gain defined by the control signal S_(G) anda sub-pass circuit including a subtracter circuit 21 and a signalprocessing circuit 26. The subtracter circuit 21 generates a differencesignal S_(R) which is processed by the signal processing circuit 26. Thesignal processing circuit 26 includes a multiplier circuit 22, a filtercircuit 23, and an amplifier circuit 24.

In the case where the signal level of the input signal S_(I) is below athreshold level, the gain "g" of the multiplier circuit 3 is set to avalue of 1. As a result, the signal level of the difference signal S_(R)is O, and thus the output signal S_(O) generated by the main passcircuit is the signal output through the adder circuit 25.

On the other hand, if the signal level of the input signal S_(I) risesabove the threshold level, the gain "g" of the multiplier circuit 3 isswitched to 1/2. As a result the rise in the input signal level isaccompanied by (1) an increase in the signal level of the differencesignal S_(R) and (2) an increase in the signal level of the outputsignal S_(S) generated by the sub-pass circuit; causing (3) theproportion of the output signal S_(S) to the output signal S_(O1) to beincreased.

Thus, by setting the input/output characteristic of the signalprocessing circuit 26 to a suitable value, it is possible to obtain agradual variation in the signal level of the output signal S_(O1) (i.e.a gentle curve increase) while the signal level of the input signalS_(I) is being increased.

Setting both the gain of the multiplier circuit 3 and the multipliercircuit 22 to "g", representing the transfer function of the filtercircuit 23 by F(ω) and representing the gain of the amplifier circuit 24by 37 A", an output signal S_(O) having an amplitude Z₁ is given by thefollowing equation:

    Z.sub.1 =gx                                                (9)

As a consequence, a difference signal S_(R) having an amplitude Z_(R) isgiven by the following equation: ##EQU3##

In addition, an output signal S_(s) having an amplitude Z₂ is given bythe following equation: ##EQU4##

Based on equations (9) and (11), an output signal S_(O1) having anamplitude y₁ can be obtained as defined by the following equation:##EQU5##

When the gains "g" of the multiplier circuits 3 and 22 are changedbetween values 0 and 1, and amplitude "x" equals one (x-1) and transferfunction F(ω) equals one (F(ω)=1 ) it is possible to predict the outputcharacteristics of the amplitude compressing/expanding circuit 20 whilethe signal level of the input signal S_(I) is kept constant and theamplitude compression control signal S_(G) is varied.

Substituting the following equations

    F(ω)=1                                               (13)

    X=1                                                        (14)

in equation (12), gives the following equation for amplitude y₁ :

    Y.sub.1 ={(1+A)-A·g}g                             (15)

FIG. 5 shows the output characteristics for the relationship where thegain "g" is varied between 0 and 1. The gain of the amplitudecompressing/expanding circuit 20 as a whole is maximum for a givenconfiguration when the gain "g" equals 0.5. The gain of the circuit as awhole increases as the gain "A"of the amplifier circuit 24 is increased.

Further, when the gain A is kept at 0 and the gain g is changed from 0to 1, the output characteristic obtained in which the gain linearlyincreases therewith, is based only on the main pass circuit. When thegain A is of values other than "0", the gains of the sub-pass circuitare superposed on the gain of the main pass circuit, and thereby, allthe output characteristics obtained have characteristic curves withascending inclinations toward the right as shown in FIG. 5.

Thus, by controlling the gain of the sub-pass circuit, the gain of theamplitude compressing/expanding circuit 20 as a whole can be controlled.Therefore, an output signal S₀₁ can be obtained which varies with adesired input/output characteristic. The characteristic based on theparameters of the sub-pass circuit for the main pass circuit.

Referring to FIG. 4, if the transfer function F(ω) of the filter circuit23 is set to a value of 1, an operating characteristic is obtainedhaving a smooth change of the amplitude compression ratio CR. Further,by setting the gain A of the amplifier circuit 24 to a specific value,it becomes possible to perform both the amplitude compressing operationand amplitude expanding operation at the same time.

The operation of the circuit of FIG. 4 is described below for the casewhere the threshold point is set to 0 dB and the amplitude compressionratio CR is 2.

In this case, the gain of the multiplier circuit 3 in the range abovethe threshold point is set to the value of x^(-1/2) according toequation (5). Hence, the gain of the multiplier circuit 22 also will beset to the value of x^(-1/2) in the range above the threshold point.

In the range below the threshold point, the amplitude x_(R) of thedifference signal S_(R) equals 0. Therefore, an output signal S₀₁proportional to the input signal S₁ as shown in FIGS. 6 and 7 isobtained.

In the range above the threshold point, by substituting

    F(ω)=1                                               (16)

    g=x.sup.-1/2                                               (17)

in equation (12), the amplitude y₁ is expressed as ##EQU6## In thiscase, if the gain A is approximately 1, and the signal level of theinput signal S_(I) is increased over the threshold point, then thevariation in the signal level of the output signal S₀₁ graduallydecreases so as to have a characteristic curve which becomes parallel tothe characteristic for an amplitude compression ratio CR of 2 (namely,parallel to the input/output characteristic represented by a straightline when the gain A is the value of 0).

Thus, it is possible according to the preferred embodiment to obtain aninput/output characteristic providing such amplitude compressing actionthat the amplitude compression ratio CR will slowly change from thevalue of 2 to the value of 2 after the signal level has crossed over thethreshold point.

In the case where the gain A becomes higher than 3, an input/outputcharacteristic is obtained in which the change in the signal level ofthe output signal S₀₁ in relation to the signal level of the inputsignal S_(I) abruptly increases when the signal exceeds the thresholdlevel. After the amplitude compression ratio CR has changed, however,from 1 to a value smaller than 1 (namely, after an amplitude expandingoperation is performed), an input/output characteristic results whicheither, gradually becomes parallel to the input/output characteristicrepresented by the straight line of the case where the gain A is 0, orcauses the amplitude compression ratio CR to approach 2.

It is apparent that the greater a change in the amplitude compressionratio, the larger the gain A to be utilized.

As a consequence, a desirable input/output characteristic can beobtained according to the preferred embodiment which provides anamplitude expansion effect after the threshold point has been crossedand then slowly turns to provide amplitude compression effect.

The case where the amplitude compression ratio CR is further increasedto 100 is examined below. In this case, CR=100 is substituted inequation (5), resulting in the following equation for "g":

    G=X.sup.-99/100                                            (19)

Substituting X^(-99/100) for x^(-1/2) in equation (18), results in thefollowing equation for amplitude y. ##EQU7##

The input/output characteristics obtained are shown in FIGS. 8 and 9 fora gain A equal to 0 and an amplitude compression ratio CR which varieslinearly in the range beyond the threshold point. When the gain A isapproximately 1, the amplitude compression ratio CR varies gently from 1to 100.

If the gain A is larger than 2, input/output characteristics areobtained which provide an amplitude expansion effect corresponding torespective values of the gain A, then slowly turn to provide theamplitude compression effect.

A description is given below of an amplitude compressing/expandingoperation in which a band-pass filter having a frequency characteristicas shown in FIG. 10 is used as the filter circuit 23 in the circuitarrangement of FIG. 4.

Thus, the filter circuit 23 is constructed of a band-pass filter circuitwhose center frequency is 2-3 KHz and is attenuating at 6 dB/OCT.

If the gain A of the amplifier circuit 24 is set to 1 and the amplitudecompression ratio CR is set to 2, the following relationship is obtainedfrom equation (12) ##EQU8##

Since the first term on the right side of equation (21) includes thetransfer function F(ω), it is known that the amplitudecompressing/expanding operation will change according to the frequencycharacteristic of the filter circuit 23.

It is known from FIG. 11 that the amplitude compression ratio CR changestoward a value of 2 as the signal level of the input signal S_(I) isincreased over the threshold point at 0 dB. Further, it is known thatthe closer to the center frequency of the band-pass filter the frequencyis, the slower the above-described change occurs.

Further, if the input signal S₁ is expressed with the frequency takenalong the abscissa as shown in FIG. 12, the amplitudecompressing/expanding operation varies in the range up to approximately15 dB of the input signal S₁. The amplitude compression ratio CR,however, becomes constant at 2 for the range where the input signal S₁exceeds approximately 20 dB.

Thus, according the preferred embodiment an amplitudecompressing/expanding circuit may be obtained having an input/outputcharacteristic with amplitude compressing/expanding action which varieswith the frequency of the input signal S₁.

Referring again to FIG. 4, the circuit is configured with the gain A ofthe amplifier circuit 24 set to approximately 1 and the center frequencyof the band-pass filter 23 at approximately 4 kHz (to obtain the highestsensitivity for a listener).

When the circuit of FIG. 4 is so configured and the parameters are setas described with reference to FIGS. 11 and 12 above, an input/outputcharacteristic is obtained providing a slowly changing amplitudecompression ratio as the signal level of the input signal is increasedover the threshold point.

Therefore, an audio signal may be obtained with a compressed dynamicrange without the unnaturalness in the reproduced sound caused in thepast by the abrupt change in the amplitude compression ratio.

Further, an input/output characteristic may be obtained providingdiffering amplitude compressing operations in accordance with thefrequencies of the input signal S₁. As a result, it is possible to causethe amplitude compression to gradually increase its effect as the inputsignal S₁ is increased in level. In addition, the frequency of the inputsignal S₁ approaches the center frequency, 4 kHz, of the band-passfilter.

Therefore, the reduction in the dynamic range near the center frequency4 kHz can be made correspondingly smaller than the reduction in thedynamic range at other frequencies.

As shown in FIG. 13, human hearing ability is most sensitive to the 4kHz frequency. Therefore, for the situation where the dynamic ranges arereduced, if the dynamic range at around the 4 kHz frequency is notreduced as much as the other frequencies, the lack of dynamic range onthe whole in the reproduced sound is not sensed to be as much of areduction than if the 4 kHz frequency range also were reduced.

Therefore, not only is the dynamic range of an audio signal obtainedfrom a digital audio signal reproducing apparatus reduced withoutinducing unnaturalness, but also the resultant feeling of reduction inthe dynamic range sensed by hearing can be reduced.

Even when an audio signal obtained from a digital audio signalreproducing apparatus is heard with background noise present, and thevolume of sound is not allowed to be made greater, or the like, thereproduced signal can be heard satisfactorily. Moreover, when the signalis recorded by an analog tape recorder, the recording can be performedwithout deterioration in the S/N ratio, distortion of the waveform andother such disturbances.

Thus, by constructing the signal processing circuit with an amplifiercircuit, a multiplier circuit, and a band-pass filter circuit, and byselecting the gain of the signal processing circuit to be apredetermined value, an input/output characteristic can be obtained inwhich the amplitude compression ratio is changed slowly as the signallevel of the input signal is increased and in which the amplitudecompression action varies with the frequencies of the input signal.

Therefore, in accordance with the preferred embodiment the dynamic rangeof an audio signal having a wide dynamic range can be compressed whilereducing the feeling of a lack of dynamic range.

Referring to FIG. 4, a low-pass filter circuit may be used in lieu ofthe band-pass filter described for the first preferred embodiment of theinvention.

In such altered configuration, an input/output characteristic can beobtained in which, as shown in FIG. 14, the higher the input signalfrequency, the lower the range of the signal level of the input signalwhere the amplitude compression action starts.

Referring to FIG. 15 a problem with a magnetic tape of an analog systemis that the output level of the recorded signal is lowered when thefrequency of the recorded signal becomes higher even if the signal levelof the recorded signal is increased higher than a specific value due toself-demagnetizing action.

By supplying the recorded signal to the recording head through theamplitude compressing/expanding circuit according to the preferredembodiments, in which the higher the frequency of the input signal is,the earlier in the rise of the signal level of the input signal that theamplitude compressing operation is started, the lowering of the outputlevel due to the self-demagnetizing action can be prevented.

Although, in the preferred embodiments described, a band-pass circuit ofa low-pass filter circuit was used for the filter circuit, the presentinvention is not limited to such examples. A high-pass filter, trapfilter, or the like also may be used as the need arises.

In addition, although in the embodiments described above the filtercircuit was used for providing the amplitude compression function so asto vary with the frequencies of the input signal, the present inventionis not so limited. The signal processing circuit also may be formed ofonly a multiplier circuit.

In such case, an amplitude compressing/expanding circuit can be obtainedhaving an input/output characteristic in which the amplitude compressionratio varies smoothly across the threshold level.

The signal processing circuit also may be provided with either only theamplifier circuit or only the filter circuit.

When only the amplifier circuit is provided an input/outputcharacteristic can be obtained by setting the gain of the amplifiercircuit to a desired value to provide an amplitude compressing functiononly when the level of the input signal comes within a specific range.

In the case where only the filter circuit is used, when the signal levelof the input signal is changed causing the signal level of thedifference signal to change, the signal level of the difference signalcomponent passed through the filter circuit to be added to the outputsignal of the main pass circuit correspondingly changes. As a result aninput/output characteristic which provides a frequency characteristicvarying with the signal level of the input signal can be obtained.

If a low-pass filter circuit is used for the filter circuit, thehigh-frequency component of the input signal can be emphasized. If aband-pass filter is used instead, the signal component of the pass bandof the band-pass filter circuit can be emphasized.

As a result, if the arrangements of the above-described first and secondpreferred embodiments are modified so as to use only the filter circuitfor signal processing effects similar to those obtained by the first andsecond preferred embodiments still may be obtained, even though simplerconstructions as a whole are used.

A clipping circuit also may be used in the signal processing circuit. Bysuch a circuit arrangement, the output signal from the signal processingcircuit can be clipped at a predetermined signal level. Correspondingly,it is possible to provide a desired change to the input/outputcharacteristic of the amplitude compressing/expanding circuit as awhole.

Although the above-described preferred embodiments were described forcases where the amplitude compression ratio CR was set to the value 1 inthe range below the threshold point, the present invention is not solimited. The amplitude compression ratio CR below the threshold pointcan be set to values other than 1. To do so the circuit parameters areadapted so that a difference signal is obtained in the signal processingcircuit when an input signal is input having a signal level above thethreshold point, whereby an input/output characteristic can be achievedfor changing the amplitude compression ratio CR smoothly from one in therange below the threshold point to a desired amplitude compressionratio.

If the amplitude compression ratio CR is set to a value less than 1 aninput/output characteristic is obtained that provides an amplitudeexpanding function in the range below the threshold point and anamplitude compressing function in the range across the threshold pointslowly changing from the amplitude expanding function.

Further, by setting the gain "A" to a value larger than 2 and using onlythe range above the threshold point, an input/output characteristic canbe obtained which provides slowly changing functions starting from anamplitude expanding function at an amplitude compression ratio CRdependent upon the gain "A" and continuing to an amplitude compressingfunction at an amplitude compression ratio CR determined by an amplitudecompression ratio control signal S_(G).

Although the above preferred embodiments were described for cases wherethe control signal is produced according to the signal level of theinput signal, the present invention is not limited to be applied to suchan arrangement, but may be applied to an amplitude compressing/expandingcircuit wherein the control signal is produced according to the signallevel of the output signal.

Further, by disposing such an amplitude compressing/expanding circuitemploying a low-pass filter circuit in front of an encoder in a digitalaudio signal processing system for the purpose of coding, the unpleasantsound to the ear due to quantization noise at the time of reproductionof female frictional sound, such as that formed by the female audiosignal when "sa" is pronounced, can be prevented from occurring.

Referring now to FIG. 16, a circuit arrangement of the control signalgenerator circuit 4 shown in FIG. 4 will be described in detail. In thiscircuit, digital information is supplied as the input signal S₁.Therefore, this control signal generator circuit performs digital signalprocessing.

In the control signal generator circuit of FIG. 16, a signal processingcircuit 112 receives a signal S2 derived from an absolute value signalS1. Signal S1 is a sequence of digital information output from anabsolute value circuit 5 into an envelope detector circuit 111. SignalS1 is processed at the envelope detector 111 to produce signal S2. Thesignal processor 112 processes signal S2 to produce normalized digitalsignals S_(OM) and S_(OE) which are output to the logarithmic convertercircuit 7 (see FIG. 3).

As shown in FIG. 17, the envelope detector circuit 111 sequentiallyreceives the absolute value signal S1, which is output from the absolutevalue circuit 5 based upon the digital audio signal S₁. The signal S1 ismade up of 16 positive bits of two's compliment data. Signal S2 is adigital product signal which rises, or falls, in a predetermined attacktime, or recovery time, dependent upon the variation in the absolutevalue signal S1.

Referring to the envelope detector circuit 111, a subtracter circuit 113receives the absolute value signal S1 and a delayed envelope detectionsignal DS2. Signal DS2 is generated by a delay circuit 114 which delaysby one clock period a digital product signal S2. The subtracter circuit113 outputs a difference signal S_(R), which is the difference betweenthe absolute value signal S1 and the delayed envelope detection signalDS2, to both a multiplier circuit 115 and a mode detector circuit 118.

A positive difference signal S_(R) is produced when the absolute valuesignal S1 is larger than that of the delayed envelope detection signalDS2, meaning that the amplitude of the digital audio signal S_(I) is inan increasing condition.

Conversely, when the difference signal S_(R) is negative, the value ofthe absolute value signal S1 is smaller than that of the delayedenvelope detection signal DS2, the amplitude of the digital audio signalS_(I) is in a decreasing condition.

A mode detector circuit 118 detects the value of the difference signalS_(R). When the value of the difference signal S_(R) is positive or zero(hereinafter to be referred to as the "attack mode"), the mode detectorcircuit 111 switches the value of an attack time control signal SA toset the gain of a multiplier circuit 115 to 0.02.

In contrast, when the value of the difference signal S_(R) is negative(hereinafter to be referred to as the "recovery mode"), the gain of themultiplier circuit 115 is set to zero.

Thus in the attack mode an output signal is obtained from the differencesignal S_(B) multiplied by 0.02, in the recovery mode, the output signalis zero.

A multiplier circuit 116 receives the delayed envelope detection signalDS2 and a recovery time control signal S_(R), then outputs an outputsignal whose value is 0.9998 times as large as that of the delayedenvelope detection signal DS2.

An adder circuit 117 adds up the output signals of the multipliercircuits 115 and 116 and outputs the summation as the digital productsignal S2. Signal S2 then is output to the signal processing circuit 112and the delayed circuit 114.

The envelope detector circuit 111 in the recovery mode can berepresented by an equivalent circuit as shown in FIG. 18. By denotingthe value of the digital product signal S2 by y(n), the value of thedelayed envelope detection signal DS2 by y(n-1), and the gain of themultiplier circuit 116 by K_(R), a relationship as expressed below isobtained:

    y(n)=K.sub.R ·y(n-1)                              (3)

As a result, even when the absolute value signal S1 changes from itsmaximum value to minimum value, a digital product signal S2 that slowlydecreases corresponding to the gain K_(R) can be obtained. And in thiscase where the gain K_(R) is selected to be 0.9998, even when the valueof the digital product signal S2 was the maximum value and the value ofthe absolute value S1 is the minimum value only bit shifting of one bitposition may be required to normalize the digital product signal S2relative to the digital product signal S2 obtained one clock periodbefore.

In practice, as illustrated in FIG. 19, since the sampling frequency ina CD player is set to 48 kHz, a recovery time of approximately 50 mseccan be obtained by selecting the gain K_(R) of the multiplier circuit116 to be 0.9998.

By contrast, in the attack mode, since the gain K_(R) is expressed as##EQU9## the circuit can be represented by an equivalent circuit asshown in FIG. 20. By denoting the gain of the multiplier circuit 115 byK_(A) and the value of the absolute value signal S1 by x(n), adifference signal S_(R) having a value Z.sub.(n) is expressed as

    Z.sub.(n) =X.sub.(n) -Y.sub.(n-1)                          (24)

Therefore, a digital product signal S2 at the output of adder circuit117 having a value y(n) is expressed as ##EQU10##

Although the value of the digital product signal S2 has the greatestchange when signal S2 was at a minimum value and an absolute valuesignal S1 is input at maximum value, normalization of S2 is achieved bybit-shifting 9 bit positions at most relative to the digital productsignal S2 obtained one clock period earlier. This is achieved by settingthe gain K_(A) of the multiplier circuit 115 to 0.02.

In practice, as shown in FIG. 21, because the gain K_(A) of themultiplier circuit 116 is set to 0.02, a value around 1 msec can beobtained which is sufficient for the attack time.

Thus, a digital product signal S2 formed of a sequence of digitalinformation can be obtained whose level varies in accordance with theamplitude of the absolute value signal S1 over the attack time andrecovery time of 1 msec and 50 msec, respectively.

Referring again to FIG. 16 the signal processing circuit 112 receivesthe digital product signal S2 at a multiplier circuit 120 which outputsa digital product signal SM based upon a control signal SC output from ashift register 122. Signal SC is the result of a bit-shifting operationin the shift register 122 wherein the digital information of the digitalproduct signal S2 is shifted by a quantity required to normalize thedigital product signal S2 received one clock period before.

The shift register circuit 122, constructed of a 15-bit reversible shiftregister circuit, responds to a bit shift signal S_(ST) output from anormalization circuit 123 to cause data of logical value "1" stored inthe shift register circuit 122 to be shifted on one bit position at atime.

For a bit shifting operation of m-bit positions the contents of theshift register circuit 122 are arranged such that the logical value ofthe m-th bit from the least significant bit (LSB) is logical "1".

The shift register circuit 122, based upon such m-th bit level, outputsa control signal SC causing the multiplier circuit 120 to generate adigital product signal SM having a value 2^(m) times as large as that ofthe digital product signal S2 input to the multiplier circuit 120.

Since bit shifting by 9 bit positions or one bit position, at most, arerequired for normalization of the information processed one clock periodbefore in the attack mode and recovery mode, respectively, the shiftingis adapted so that input digital information having a bit length 9 bitslonger (i.e 25 bits) than the bit length of the information input to themultiplier circuit 120 (i.e. 16 bits) is output to the normalizationcircuit 123.

Thus, the digital information forming the digital product signal SM andinput to the normalization circuit 123 first is shifted "m" bitpositions to normalize the digital information relative to the digitalinformation processed one clock period before.

The normalization circuit 123 receives a mode detection signal S_(D)from the mode detector circuit 11B of the envelope detector circuit 111to identify either the attack mode or the recovery mode. The circuit 123normalizes, as shown in FIG. 22, the digital product signal SM accordingto the mode detection signal S_(D) so that the logical value of thesecond bit from the most significant bit (MSB) of a 16-bit data word isat a logical "1" (FIG. 22(A)).

More particularly, in the attack mode the digital product signal SMinput to the normalization circuit 123 is shifted so that the logicalvalues of the bits in the two highest positions of the 25-bit long dataword are logical "0" and "1" (FIG. 22 (B)) or are both "0" (FIG. 22(C)).

Then, the normalization circuit 123 detects the logical value of thesecond bit from the most significant bit, and shifts the digitalinformation toward the higher order until the logical value of the bitin question is at a logical "1".

Further, the circuit 123 outputs the bit shift signal S_(ST) (see FIG.16) corresponding to the number of bits shifted and at the same time,outputs the most significant 16 bits to the logarithmic convertercircuit 7 (FIG. 3) as the mantissa of envelope detection signal S_(OM).

In the recovery mode, as shown in FIG. 23, the logical values of thedigital product signal SM bits from the most significant bit to thetenth bit of the 25-bit long data are logical "0" while the logicalvalue of the 11th bit is logical "1" (FIG. 23(A)). Alternatively, thelogical values of the bits from the most significant bit to the 11th bitare logical "0" in succession and the logical value of the 12th bit islogical "1" (FIG. 23 (B)).

The normalization circuit 123, in the recovery mode, detects the logicalvalue of the 11th bit from the most significant bit. When the logicalvalue of that bit is logical "0", bit shifting a bit shift of one bitposition toward the most significant bit occurs.

The normalization circuit 123 then outputs the 16 least significant bitsas the exponent of the normalized data. Thus, the exponent of thenormalized envelope detection signal S_(OM) in both attack mode andrecovery mode can be obtained.

Because of the prior bit shifting by the bit-shift quantity "m" requiredfor normalization of the digital product signal SM processed one clockperiod before, normalization of the digital product signal SM in theattack mode is achieved by shifting 9 bit positions at most andnormalization of the digital product signal SM in the recovery mode isachieved by shifting one bit position at most.

Therefore, as compared with the prior art case where digital informationis sequentially normalized, the number of bit shifts in the present casecan be reduced according to the shifting performed one clock periodearlier for normalization.

As a result, the processing time required for the bit shiftingcorrespondingly is shortened and the processing is simplified, enablinga signal processing circuit of a simplified circuit arrangement.

Referring to FIG. 16, a counter circuit 124, formed of a reversiblecounter, increases or decreases the count value obtained one clockbefore in response to the bit shift signal S_(ST). As a result, theexponent of the envelope detection signal S_(OE) corresponding to themantissa of the envelope detection signal S_(OM) obtained one clockperiod before is updated and output to the logarithmic converter circuit7 (FIG. 3).

At the same time, the shift register circuit 122 shifts the bit oflogical value "1" according to the bit shift signal S_(ST) and recordsthe bit shift quantity required for normalization of the digitalinformation in the current clock period.

In the described manner, the envelope detection signals are output asexponent signal S_(OE) and mantissa signal S_(OM) and passed through thelogarithmic converter circuit 7, clipping circuit 8, multiplier circuit10, and the exponential converter circuit 11 (FIG. 3). As a result, aspecified control signal S_(G) can be obtained and thus, a controlsignal generator circuit is achieved for producing small calculationerror as a whole with high speed processing in a simplified circuitarrangement.

In the described circuit arrangement, the digital audio signal S₁ isconverted into the absolute value signal S1 and then input to theenvelope detector circuit 111.

As a result, the digital product signal S2 varying in predeterminedattack time or recovery time is obtained through the envelope detectorcircuit 111.

In the signal processing circuit 112, the digital product signal S2 issubjected to bit shifting by (1) the bit-shift quantity required fornormalizing the digital product signal S2 received one clock periodbefore, and (2) the bit shifting in the normalization circuit 123according to whether in the attack mode or recovery mode.

As a result, the mantissa and exponent information of the digitalproduct signal S2 are obtained in the normalization circuit 123 and thecounter circuit 124, respectively, and output from the signal processingcircuit 112.

According to the above-described circuit arrangement, a signalprocessing circuit performing the signal processing in a simple mannerand shorter processing time with a simpler circuit arrangement as awhole can be obtained by virtue of the normalization of signal S2performed one clock period earlier.

Therefore, a control signal generator circuit of a correspondinglysimpler circuit arrangement as a whole, producing smaller errors, andcapable of speedier operational processing can be obtained.

Although the above embodiment was described for the case where theattack time and the recovery time are set to 1 msec and 50 msec, thepresent invention is not limited thereto, but may be widely applied toother cases in which a digital signal varying in attack time andrecovery time is subjected to a normalization process in floating pointarithmetic.

For a case where the attack time and recovery time are changed, the bitlength of the digital information input to the normalization circuit maybe suitably selected in accordance with the changed attack time andrecovery time. The corresponding bit shifting processes then areperformed in the normalization circuit.

Subsequently, a control signal generator circuit according to anotherpreferred embodiment will not be described with references to FIG. 24.

In FIG. 24, an input signal S₁ composed of a digital audio signal havingan amplitude "X" is supplied via an absolute value circuit 5 to anenvelope detector 6. A detected signal S_(L) is output from the envelopedetector circuit 6 having a signal level proportional to the signallevel "X" of the input signal S_(I). Multiplier circuits M₁, M₂,M₃, - - - , M_(N) (N being an integer) are connected in series to formN-series-connected multiplier circuits. Each multiplier circuit receivesthe detected signal S_(L) at its respective input terminal. The detectedsignal S_(L) controls the gain of each respective multiplier circuit.The detected signal S_(L) also is supplied at the multiplier circuit M₁to the N-series-connected multiplier circuits.

Assuming that the signal level of the detected signal S_(L) is in directproportion to the signal level "X" of the input signal S_(I) at aproportion constant of 1, a signal level "Y₁ " of a multiplier signalS_(M1) is obtained from the multiplier circuit M₁ and expressed as:

    y.sub.1 =x.sup.2                                           (26)

In the succeeding multiplier circuit M₂ a signal level "y₂ " ofmultiplier signal S_(M2) is obtained as represented by the followingequation: ##EQU11##

For each of multiplier circuits M₁ through M_(N), the signal levels y₁,y₂, y₃, - - - , y_(N) of the respective multiplier signals S_(M1),S_(M2), S_(M3), - - - , S_(MN) can be produced as expressed by thefollowing generic formulae: ##EQU12##

With the above-described arrangement, both the absolute value circuit 5and envelope detector circuit 6 constitute the detector means fordetecting the signal level of the input signal S_(I). The multipliercircuits M₁ through M_(N) constitute a multiplier means for performingmultiplication on the detector results at a predetermined multiplicationnumber with a coefficient defined in accordance with the detectorresults obtained from the detector means.

Referring again to FIG. 24, the control signal generator circuit alsoincludes (N+1) sets of weighting circuits A₀, A₁, A₂, A₃, - - - , A_(N)respectively constructed of amplifier circuits. The signal S_(L) and therespective multiplier signals S_(M1), S_(M2), S_(M3), - - - , S_(MN) areinput to respective weighting circuits. The outputs of each weightingcircuit are input to the adder circuit 229.

Accordingly when the weighting coefficients of the respective weightingcircuits A₀, A₁, - - - , A_(N) are express by values C₀, C₁, C₂,C₃, - - - , C_(N1) an output signal S_(A0) having a signal level y_(A0)can be obtained via the first weighting circuit A₀ which is representedby the following equation:

    Y.sub.A0 =C.sub.0 x                                        (30)

In the remaining weighting circuits A to A_(N=l) , output signalsS_(A1), S_(A2), S_(A3), - - - , S_(AN) having a signal level Y_(A1),Y_(A2), - - - Y_(AN), respectively, can be obtained which are generallyexpressed by the following equation: ##EQU13##

The adder circuit 229 receives the above-described output signals S_(A0)to S_(AN), along with a signal S_(A-1) having a signal level Y_(a-1)derived from a coefficient generator circuit 228 (the signal level ofwhich corresponds to a signal level expressed by a value C₋₁corresponding to the output signals S_(A0) to S_(AN)). The adder circuitoutputs a resultant adder signal as a control signal S_(G1).

As a consequence, a control signal S_(G1) having a signal level Y_(G1)is produced as an output from the adder circuit 229 based upon theabove-described equations (30) and (31), as expressed by the followingequations. ##EQU14##

As previously described in detail, the weighting circuits A₀ to A_(N)constitute weighting means for weighting the detected signal S_(L) andthe multiplied signals S_(M1) to S_(MN) by predetermined weightingamounts, whereas the adder circuit 229 constitutes adder means forsumming the output signals S_(AO) to S_(AN) obtained by the weightingoperations.

Consequently, when the amplitude compressing/expanding circuit isconstructed of the above-defined circuit arrangement by setting theweighting circuits A₀ to A_(N) and the weighting coefficients C₁ toC_(N) of the coefficient generator circuits 228 to predetermined values,the input/output characteristics indicated by the high order functionformula can be obtained. As a result, input/output characteristics canbe obtained with a simple circuit arrangement while having an amplitudecompression ratio which varies complexly.

Referring to FIG. 25, another preferred embodiment of the multipliercircuit 3 and control signal generator 4 shown in FIG. 4 will bedescribed.

In FIG. 25, reference numeral 320 represents both the multiplier circuit3 and the control signal generator 4 illustrated in FIG. 4. This circuitarrangement receives, as an input signal, a voice signal S_(I) at anon-inverting input terminal of an operational amplifier circuit 321.The amplifier circuit 321 has a gain of more than 50 dB and provides atthe output terminal an amplitude compressed, expanded, or expanded andcompressed output signal S₀₁.

The operation amplifier circuit 321 uses a resistor 322 functioning as afirst feedback circuit for maintaining the amplifier gain at a constantvalue even when the signal level of the output signal S₀₁ is changed.The operational amplifier circuit 321 uses a second feedback circuitwhich is constructed of a multiplier circuit 323 and an envelopedetector circuit 324. The multiplier circuit 323 has an amplitude gainwhich is increased when the signal level of the output signal S₀₁ isincreased, while the envelope detector circuit 324 controls theamplitude gain of this multiplier circuit 323.

More specifically, the envelope detector circuit 324 responds to theoutput signal S_(L1) to produce a control signal S_(G1) in directproportion to the output signal S_(L1) so as to control the amplitudegain of the multiplier circuit 323. As a result, the input/outputcharacteristics of the multiplier circuit 323 can be changed in responseto the signal level of the output signal S₀₁.

The output signal S₀₁ of the multiplier circuit 323 is supplied via aresistor 325 to the operational amplifier circuit 321 at thenon-inverting input terminal together with the output signal S_(L2) ofthe resistor 322. Thus, a summation signal derived from the first andsecond feedback circuits is fed back to the operational amplifiercircuit 321. A resistor 326 connects the non-inverting input terminal toground.

Representing circuit 320 with an equivalent circuit, the envelopedetector circuit 324 constitutes a control signal generator circuit 329,while the multiplier circuit 323 constitutes an amplitude modulatorcircuit 330, the two circuits together representing a second feedbackcircuit 331. The amplitude gain of the modulator circuit 330 is variedin accordance with the control signal S_(G1) output from the controlsignal generator 329. The first feedback circuit 322 of FIG. 25 isconstituted by a first feedback circuit 332 in FIG. 26.

The resistor 326 constituted by an adder circuit 333 for adding theoutput signals S_(L1) and S_(L2) of the first and the second feedbackcircuits 332 and 331, respectively. The operational amplifier circuit321 of FIG. 25 is constituted by a subtracter circuit 334 whichsubtracts the output signal of the adder circuit 333 from the voicesignal S₁, along with an amplifier circuit 335 having a predeterminedgain.

Accordingly, the circuit 320 may be constructed of a feedback amplifiercircuit including two feedback circuit 331 and 332, so that theinput/output characteristics of both the first and second feedbackcircuits 332 and 331 determine those of the entire circuit 320.

It should be noted in the circuit 320 shown in FIG. 25 that the feedbackgains of the first and second feedback circuits may be equal for a voicesignal S_(I) having a predetermined signal level. Simultaneously, thetotal gain of the circuit 320 may be set to a value of 1 in the firstfeedback circuit 332.

As a consequence, input/output characteristic for circuit 320 can beobtained which are determined by the input/output characteristic of thefirst and second feedback circuit 332 and 331 at the boundary of thesignal level. With an increase in the signal level of the voice signalS_(I) at the boundary of the signal level, the signal level of theoutput signal is increased, and the amplitude gain of the multipliercircuit 323 is correspondingly increased, so that the circuit 320 can bedetermined from the first feedback circuit 332 and second feedbackcircuit 321. Accordingly, if the input signal level is set to the signallevel obtained at the threshold point, the input/output characteristicof circuit 320 becomes the input/output characteristic determined by thefirst and second feedback circuits 332 and 331 at the boundary of thethreshold point.

In addition, since the summation signal derived from the first andsecond feedback circuit 332 and 331 is fed back, the input/outputcharacteristic determined by the first feedback circuit 332 at theboundary of the threshold point, can be varied to equal the input/outputcharacteristic determined by the second feedback circuit 331.

Accordingly, for selected input/output characteristics of both the firstand second feedback circuits 332 and 331, the input/outputcharacteristic of the circuit 320 can be smoothly changed from theinput/output characteristic determined by the first feedback circuit 332to the input/output characteristic determined by the second feedbackcircuit 331.

Equating the amplitude of the output signal S₀₁ to a value of x₁, theamplitude of the output signal S_(L1) derived from the multipliercircuit 323 to a value of y₁, and the gain of the multiplier circuit 323to a value of g₁, y, then can be expressed as:

    y.sub.1 =g.sub.1 x.sub.1                                   (33)

If in the envelope detector circuit 324, the proportion constant isequal to a value of "m", g, can be expressed as:

    g.sub.1 =m y                                               (34)

Now, if the proportional constant "m" is equal to a value of 1, thefollowing equation is obtained from the equations (33) and (34);

    y=y x                                                      (35)

Accordingly, when the equation (35) is simplified, the followingequation is given;

    x=1                                                        (36)

As a consequence, as illustrated in FIG. 27, the input/outputcharacteristic for the amplitude gain of the second feedback circuit 331can be obtained in which the output level is changed to infinity withrespect to the input level.

To the contrary, as shown in FIG. 28, in the first feedback circuit 332,an input/output characteristic can be obtained in which the output levelis changed in direct proportion to the input level at a proportionalconstant of 1.

As a result, the input/output characteristic illustrated in FIG. 29 canbe obtained at the boundary of the input level at which both the outputlevels of the first and second feedback circuits 332 and 331 becomeequal. That is, the input/output characteristic is smoothly changed fromthe input/output characteristic of the first feedback circuit 332, asdescribed in FIG. 23, to that of the second feedback circuit 331 inaccordance with an increase of the input level.

Consequently, the input/output characteristic of the entire circuit 320is expressed by the reverse characteristic of the first and secondfeedback circuits 322 and 321, as illustrated in FIG. 30. The amplitudecompression ratio CR can be smoothly varied with respect to the boundaryof the threshold point (0 dB at the input signal in the preferredembodiment), from a value of 1 represented by the proportional constantof "1" to the infinite value represented by the proportion constant of"0".

Moreover, in the preferred embodiment, the first feedback circuit 332 isconstructed of a resistor circuit, and the second feedback circuit 331is constructed of the envelope detector circuit 324 for controlling thegain of the multiplier circuit 323. As a result the amplitude gains ofthe first and second feedback circuits 332 and 331 can be set with anextremely high accuracy.

As a consequence, the signal level at the threshold level can be setwith a high accuracy, and since the entire circuit is constructed of thefeedback circuits, the higher stability can be achieved as a whole, forthe temperature, supply voltage and so on.

In a practical case, according to the conventional amplitudecompressing/expanding circuit, the signal level at the threshold pointmust be set by the circuit network employing the non-linear circuitelement such as the clipping circuit. As a result, there is no effectivesolution to prevent the fluctuation in the operation level of thenon-linear circuit element. Also there is a problem that the temperaturecharacteristics are not sufficiently stable.

However, according to the preferred embodiment of the invention, theseconventional problems can be effectively solved.

With the above-described circuit arrangement, for the case where thesignal level of the voice signal S_(I) is lower than the thresholdpoint, the entire input/output characteristic of the circuit 320 can beset by the amplitude gain of the resistor 322 to achieve theinput/output characteristic where the amplitude compression ratio CR isequal to a value of 1.

When the signal level of the voice signal S_(I) is gradually increased,the amplitude compression ratio CR is gradually changed due to theamplitude gain of the second feedback circuit 331. When the signal levelof the voice signal S_(I) exceeds the threshold point, the input/outputcharacteristic where the amplitude compression ratio CR becomes infinitecan be obtained through the second feedback circuit 331.

According to the above-described circuit arrangement, the secondfeedback circuit is employed with respect to the first feedback circuitso that the entire circuit can be simply constructed, and theinput/output characteristic is obtained in which the amplitudecompression ratio CR is smoothly varied from the value of 1 to theinfinitive value.

In addition, according to the preferred embodiment of the invention,since the entire input/output characteristic of the circuit 320 can bedetermined by the summation signal of the first and second feedbackcircuits the amplitude compression ratio CR can be changed with respectto the desired threshold point. As a consequence, the threshold pointcan be set with high accuracy relative to the conventional circuitarrangement, and an amplitude compressing/expanding circuit is obtainedhaving stable temperature characteristic.

Referring to FIG. 31, a circuit arrangement is described in which thecircuit of FIG. 25 is employed in the preferred embodiment of FIG. 4.

The same reference numerals are employed in FIG. 31 for denoting thesame of similar circuit elements of FIG. 25. Reference numeral 340indicates as a whole an amplitude compressing/expanding circuit. A mainsignal pass circuit 320 is constructed of the circuit 320 shown in FIG.25. A signal sub-pass circuit is employed with the main pass circuit.

In the signal sub-pass circuit a subtracter circuit 341 receives theoutput signal S₀₁, from the operational amplifier circuit 321 of themain signal pass circuit 320 and the voice signal S_(I) to generate adifference signal S_(R) received by a non-inverting input terminal of anoperational amplifier circuit 342.

The operation amplifier circuit 342, which is similar to the operationalamplifier circuit 321, uses a first feedback circuit and a secondfeedback circuit. The first feedback circuit is constructed of aresistor 343. The second feedback circuit is constructed of a multiplier344, an envelope detector 345 and a resistor 346. The envelope detectorcircuit 345 controls the gain of the multiplier circuit 344. The firstand second feedback circuit outputs are summed and input to theinverting input, which is connected to ground through resistor 347. Theoperational amplifier circuit 342 outputs signal 342 which is fed backto the inverting input terminal via the resistors 343 and 346.

The envelope detector circuit 345 receives the output signal S_(L1) ofthe multiplier circuit 323 of the main pass circuit and charges the gainof the multiplier 344 based on such signal S_(L1) similar to the controlof the gain for multiplier 323 of the main pass circuit.

Furthermore, the resistance values of the resistors 343, 346 and 347 areselected in such a manner that the input/output characteristic of theoperational amplifier circuit 342 is equal to the input/outputcharacteristic of the operational amplifier circuit 321.

The adder circuit 350 of FIG. 31 outputs a summation signal of theoutput signals S₀₁ and S₀₂ of the operational amplifier circuits 321 and342 as the output signal S₀₃ of the amplitude compressing/expandingcircuit 340.

Referring to FIG. 32, the circuit arrangement of FIG. 31 is expressed asan equivalent circuit. Comparing FIGS. 26 and 32, like reference numeraldenote like circuit elements. With regard to the circuit of FIG. 31 thecircuit 320 is constituted by the main pass circuit 359, whereas the inFIG. 31 comprising elements 342-347 is constituted by a sub-pass circuit370 and amplifier 371.

With respect to the sub-pass circuit in FIG. 31, the resistor 343 isconstituted by the first feedback circuit 361 in FIG. 32, the multipliercircuit 344 is constituted by the amplitude modulator circuit 362 andthe envelope detector circuit 345 is constituted by the control signalgenerator circuit 363. Thus the amplitude modulator circuit 362 and thecontrol signal generator circuit 363 constitute a second feedbackcircuit 365.

The resistors 346 and 347 are constituted by the adder circuit 366. Theoperational amplifier circuit 342 is constituted by subtracter 358 andamplifier 360. The gain of the sub-pass circuit 370 is changed like thegain in the main pass circuit 359, (i.e. in response to the input signalS_(I) and output signal S₀₁ of the main pass signal 359, and thedifference signal S_(R).) The amplifier circuit 360 output signal, viathe amplifier circuit 371 having a predetermined gain, is supplied toadder circuit 350 where it is added to the output signal S₀₁ of the mainpass circuit 359 to provide a summation signal S₀₃.

As a consequence, because control signal S_(G1) is produced in controlsignal generator circuits 329 and 363 based upon the output signalS_(L1) of the amplitude modulator circuit 330 of the main pass signal359, the circuit arrangement shown in FIG. 32 is equivalent to thecircuit formed by eliminating the filter circuit 23 from the circuitarrangement shown in FIG. 4 when the main pass circuit 359 and thesub-pass circuit 370 are represented as the multiplier circuit, theinput/output characteristic of which is changed in response to thesignal level of the input signal.

In other words, both the main pass circuit 359 and sub-pass circuit 370are represented as the multiplier circuits 375 and 376, respectively.The gains of these multiplier circuits 375 and 376 are changed by meansof the control signal S_(G2). The amplitude compression ratio CR then isequal to a value of 1 in the range below the threshold point, and isequal to an infinite value in the range above the threshold point.

Referring now to FIG. 33, a still further preferred embodiment of theinvention will be described by changing the representing method andmoreover the circuit arrangement of the embodiment shown in FIG. 4.

In FIG. 33, reference numeral 420 represents an amplitudecompressing/expanding circuit. An input signal S_(I) constructed of adigital audio signal is received by a control signal generator circuit421, a multiplier circuit M₁, and a weighting circuit A_(o).

The control signal generator circuit 421 is operated in such a mannerthat the input signal S_(I) is received at an absolute value circuit 423for full wave rectifying the input signal. The rectified signal signedthen is input to an envelope detector circuit 424 then a reciprocalinverting circuit 425 which outputs a detected signal S_(L) directingproportional to the input signal as a control signal S_(G1).

The control signal S_(G1) is supplied from the signal generator 421 witha signal level Y_(G1) expressed by the following equation (37) in whichproportion constant "a₁ " represents the proportion of the amplitude xof the input signal "S_(I) "; ##EQU15##

The control signal S_(G1) is supplied to N-series-connected multipliercircuit M₁, M₂, M₃, . . . , M_(N) so as to control gains of eachseries-connected multiplier circuit M₁ through M_(N).

As a result, the respective multiplier circuits M₁ to M_(N) performmultiplier processing on the input signal by utilizing a gain equal to acoefficient x⁻¹ as represented by the below-mentioned equation (38):

    g=x.sup.-1                                                 (38)

At multiplier circuit M₁ signal S_(I) is input, denoted by the followingequation:

    y.sub.0 =x                                                 (39)

At multiplier circuit M₁, the multiplied signal S_(M1) with amplitude Y₁is obtained as an output represented by the following equation (40);##EQU16##

Because the multiplied signal S_(M1) is supplied into the succeedingmultiplier circuit M₂ to be multiplied therein, a multiplied signalS_(M2) having amplitude y₂ denoted by the following equation (4) isobtained: ##EQU17##

Thus, in the multiplier circuits M₁ M_(N), the input signal issequentially multiplied so that the multiplier processing is carried out"N" times corresponding to the N-series connected multiplier circuits.In the respective multiplier circuits M₁ M_(N), the amplitudes of themultiplier signals S_(M1), S_(M2), S_(M3), . . . , S_(MN) can beobtained which are represented by the following equation: ##EQU18## Whensimplifying this generic equation (42), the multiplied signals can beexpressed by the following equation:

    Y.sub.n =X.sup.-(n-1)                                      (43)

In addition, the amplitude compressing/expanding circuit 420 includes(N+1) weighting circuits A₀, A₁, A₂, A₃, . . . , A_(N) made of amplifiercircuits. Weighting circuit A₀ receives the input signal S_(I) while theremaining weighting circuits receive, respectively multiplier signalsS_(M1), S_(M2), S_(M3), . . . , S_(MN). Moreover, the amplitudecompressing/expanding circuit 420 weights both the input signal S_(I)and the respective multiplied signals S_(M1) to S_(MN) by weightingcoefficients determined by the gains of amplifiers forming therespective weighting circuits A₀ to A_(N). The output signal S₀₁ of theamplitude compressing/expanding circuit 420 is the summation of signalsS_(A0) to S_(AN).

Expressing the weighting coefficients of the respective weightingcircuits A₀ to A_(N) by the respective values C₀, C₁, C₂, C₃, . . . ,C_(N), enables the amplitude of the respective weighting circuit outputsignals to be expressed in terms of the weighting coefficients. Theoutput signal S_(A0) may be expressed by the following equation:##EQU19##

In the remaining weighting circuits A₁, to A_(N), the output signalsS_(A1), S_(A2), S_(A3), . . . , S_(AN) can be represented by thefollowing generic equation: ##EQU20##

As a consequence, the output signal S_(o1) can be obtained via the addercircuit 429 from the previous equations (44) and (45), as represented bythe following equation: ##EQU21##

Accordingly since the weighting coefficients C₀ to C_(N) of theweighting circuits A₀ to A_(N) are set to the desired values, Theinput/output characteristics expressed by the higher order functionformula of the equation (46) can be achieved, and an input/outputcharacteristic is obtained in which the amplitude compression ratio issmoothly and complexly varied with a simpler circuit arrangement.

In the practical case, when both the weighting coefficients C₀ and C₁ ofthe weighting circuits A₀ and A₁ are set to a value of 1, and theweighting coefficients C₂ and C_(N) of the weighting circuits A₂ toA_(N) are set to a value of 0, the output signal S₀₁ can be representedby the following relation equation (47) as derived from the previousequation (46):

    y.sub.01 =x+1                                              (47)

In such case, as illustrated in FIG. 34, where the amplitude compressionratio CR is equal to both x and the value of 1, as represented by thefollowing relationships: ##EQU22## the amplitude compressing/expandingcircuit can be obtained where the amplitude compression ratio issmoothly changed from x to the value of 1 at the threshold pointcorresponding to the input level at which the first mentionedinput/output characteristic is intercepted. The input/outputcharacteristic of the amplitude compressing/expanding circuit then isexpressed by the following equations: ##EQU23##

With the above-described circuit arrangement, the control signal S_(G1)is obtained from the control signal generator circuit 421 based upon theinput signal level of the input signal S_(I). The series-connectedmultiplier circuits M₁ to M_(N) perform the multiplying process on theinput signal S_(I) with the coefficient x⁻¹, based upon this controlsignal.

As a result, the multiplied signals S_(M1) to S_(MN) are obtained fromthe multiplier circuits M₁ to M_(N) representing the respective terms ofthe (N-1) order function formula with respect to the amplitude "X" ofthe input signal S_(I). These multiplied signals are summed with theinput signal S_(I) via the predetermined weighting coefficients C₀ toC_(N) of the weighting circuits A₀ to A_(N) and then output, so that theinput/output characteristic represented by the (N-1) higher orderfunction formula can be obtained.

With the above-described circuit arrangement, the multiplier circuitsare series-connected and the multiplied outputs are added to thepredetermined coefficients so as an input/output characteristicrepresented by the higher order function formula with respect to theamplitude of the input signal can be obtained. As a consequence, anamplitude compressing/expanding circuit can be obtained with simplercircuit arrangement, in which the amplitude compression ratio issmoothly and complexly changed.

We claim:
 1. An amplitude expanding or compressing circuit for expandingor compressing an amplitude of an input signal, comprising;controlsignal generating means for generating a control signal in response tothe input signal; signal multiplier means for multiplying the controlsignal and said input signal to generate a multiplied output signal;first subtracter means for subtracting said input signal from saidmultiplied output signal to generate a first subtracter output signal;signal processing means receiving said first subtracter output signalfor generating a processed output signal from said first subtracteroutput signal; and second subtractor means for subtracting saidprocessed output signal from said multiplied output signal to generatean amplitude expanded or compressed output signal.
 2. The circuitaccording to claim 1, wherein said signal processing circuit includes asecond signal multiplier means for multiplying said control signal bysaid first subtracter output signal.
 3. The circuit according to claim2, wherein said signal processing means further includes amplifier meansreceiving the output of said second multiplier means, the output of saidamplifier means being said processed output signal.
 4. The circuitaccording to claim 1, wherein said signal processing means includesfilter means receiving the output of said first subtracter means forgenerating a filtered output signal.
 5. The circuit according to claim4, wherein said filter means includes a low-pass-filter.
 6. The circuitto claim 1, wherein said control means includes:an absolute value meansreceiving said input signal for generating absolute value output signal;envelope detector means for detecting an envelope of said absolute valueoutput signal for generating a detected output signal; a signalmultiplier having n plural stages, each of said stages having tworespective input terminals and a respective output terminal, a first oneof said stages receiving said detected output signal at said tworespective terminals, the remainder of said stages each receiving, atsaid two respective input terminals thereof, an output signal from saidoutput terminal of the preceding stage and said detected output signalof said envelope means; a plurality of weighting circuits having n+1circuits in first circuit multiplies a predetermined coefficientdetected output signal and in which, each remaining circuit multiplies arespective predetermined coefficient by said output signal of thepreceding stage for respective output signals; and adder means foradding said respective output signals of said circuits together.
 7. Thecircuit according to claim 1, wherein said control generating means andsaid signal multiplier means together comprise an operational amplifierfor supplying an amplified output signal having first and secondfeed-back circuits, said feed-back circuit including a voltage dividerfor said amplified output signal of said operational amplifier toprovide a voltage divided output signal which is fed to an input of saidoperational amplifier, said second feed-back circuit including amultiplier receiving said amplified output signal of said operationalamplifier, an envelope detector for detecting an envelope of an outputsignal of said multiplier, an output signal of said envelope detectorbeing output to said multiplier, and the output signal of saidmultiplier being further input to said operational amplifier.